Device and method for producing nanoparticles

ABSTRACT

The invention relates to a device and a method for producing nanoparticles, in which method starting materials for nanoparticles are mixed at least as liquid droplets and optionally also as gases and/or vapors with at least combustion gases in a premixing chamber and the mixture is separated for liquid drops larger than size d, whereafter the mixture is conducted to at least one burner, in which the combustion gases are ignited such that a heavily mixing flame is generated, in which the starting materials react and optional solvents evaporate and generate through nucleation and/or sintering and/or agglomeration particles having a diameter of 1 to 1000 micrometers.

FIELD OF THE INVENTION

The invention relates to a device in accordance with the preamble ofclaim 1 and, in particular, to a device for producing nanoparticles, thedevice producing liquid droplets containing starting materials fornanoparticles, which liquid droplets are conducted to a thermal reactorin which the nanoparticles are formed from the liquid droplets.

DESCRIPTION OF PRIOR ART

Nanoparticles, i.e. particles having a size of 1 to 1000 nanometers,have been found to have a plurality of significant applications, such ascatalytic surfaces, self-cleaning and antibacterial products, glassdyeing, sunscreen lotions and manufacturing of optical components, suchas an optical fibre, etc. Feasible production of nanoparticles is acrucial factor in view of the feasible use of these applications.Relatively narrow size distribution (mono-dispersivity),anti-agglomeration and homogeneity are required of the nanoparticles.Nanoparticle production should be readily convertible fromlaboratory-scale production to industrial-scale production.Nanoparticles may be produced both by wet chemical processes and byvapour phase processes, of which the vapour phase processes aregenerally simpler and more readily scalable than the wet processes. Thevapour phase processes, also known as aerosol reactor processes, includeflame reactors, hot-wall reactors, plasma reactors, gas condensationmethods, laser ablation and spray pyrolysis among other things. Theflame reactor and the spray pyrolysis process represent the prior artessential to the present invention. The prior art is set forth, forinstance, in KONA, 2004, No. 22, L. Mädler, “Liquid-fed Aerosol Reactorsfor One-step synthesis of Nano-structured Particles”, p. 107-120. Thearticle is briefly summarized below to present the prior art.

The spray processes for producing nanoparticles differ from one anothermainly in the manner how the thermal energy necessary for pyrolysis isintroduced into the process. Introduction of the thermal energy affects,for instance, the maximum temperature, temperature profile and residencetime. The four principal methods for spray processes in nanoparticleproduction include spray pyrolysis in tubular reactor (SP), vapour flamereactor spray pyrolysis (VFSP), emulsion combustion method (ECM) andflame spray pyrolysis (FSP). Of these methods the SP employs a hot-wallreactor as the thermal reactor and therefore it is not relevant to thepresent invention. ECM and FSP use oil and exothermic liquid as fuel andtherefore they are not relevant in connection with the presentinvention.

The vapour flame reactor uses as heat source a thermal reactor providedby means of combustion gases. A considerably higher temperature and ashorter residence time are the advantages of the flame over the hot-wallreactor. In the VFSP reactor the raw material is vaporized with abubbler or an evaporator and the vapour is conducted to the flameprovided by means of the combustion gases. The vapours may be mixed withthe combustion gases either upstream of a pre-mixed burner or outsidethe burner. The raw materials react in the flame and form particles.Scarcity of raw materials is a disadvantage of the process. Only fewelements have compounds with sufficiently high vapour pressure for theprocess.

The process has been further developed to include modifications in whichliquid raw materials are atomized and fed into the flame. Thesemodifications are set forth in U.S. Pat. No. 3,883,336 A, U.S. Pat. No.5,876,683 A, U.S. Pat. No. 6,447,848 A, in US patent application U.S.2002/0031658 A1 and in Finnish patent FI98832 B.

U.S. Pat. No. 3,883,336 discloses an apparatus, in which silicontetrachloride is passed in a form of mist into a flame spray by means ofoxygen acting as a carrier gas. Said publication further disclosesspraying aerosol into the flame of the flame spray from outside toproduce glass. Said apparatus has poor efficiency and passing silicontetrachloride as vapour into the apparatus by means of a carrier gas isslow, because if the amount of silicon tetrachloride is excessive inrelation to the carrier gas, it nucleates into larger droplets, andconsequently, sufficiently small particles will not be obtained byspraying.

U.S. Pat. No. 5,876,683 discloses a method for the production ofnanomaterial. The disclosed method produces nanoparticles from gaseousstarting materials at a reduced pressure, typically of 1 to 50 mbar. Themethod being limited to the use of gaseous starting materials excludesquite a large number of starting materials and the use of organometalsas starting materials makes the process expensive.

U.S. Pat. No. 5,958,361 discloses a method for the production ofnanosized material by a spray pyrolysis process. In the process astarting material dissolved in an organic solvent is fed into the flame,where the substances react and produce nanosized particles. Combustionof the solvent produces the majority of the energy required for thereactions. The patent does not disclose a method for droplet formationof liquid starting materials into a premixing chamber, but droplets areformed directly into the flame, whereby the process is substantiallyless controllable than in the present invention.

U.S. Pat. No. 6,447,848 discloses a method of producing fine-structuredcoatings by employing thermal spraying, in which coating raw materialsare fed in liquid form into the flame. The patent does not disclose amethod for droplet formation of liquid starting materials into apremixing chamber, but liquid starting materials are fed directly intothe flame, whereby the process is substantially less controllable thanin the present invention. The invention of said patent has been furtherdeveloped in US patent application 2002/0031658, which does not describedroplet formation into a premixing chamber either.

Finnish patent FI98832 discloses a method and an apparatus, in which asubstance to be sprayed is conducted in liquid form into the flame andatomized by means of a gas substantially in the vicinity of the flamesuch that atomization and flame generation take place in the samedevice. Further, said publication sets forth that the device comprisesmeans for conducting a liquid substance into the flame and means forconducting the gas into the liquid to be sprayed such that the gassprinkles the liquid to be sprayed into droplets substantially in thevicinity of the flame, whereby droplets are formed in the same devicewith the flame. Speeds of the sprinkling gases and the combustion gasesto be used in said method may differ considerably from one another,which may cause refluxes in the flame to be generated and fouling andeven clogging of the burner resulting from the refluxes. Simultaneoussprinkling of a plurality of different liquids is difficult in saidmethod. The scalability of the method and the apparatus is cumbersome,because each burner requires a separate adjustment of gas stream ratesto enable good control of atomization and flame generation.

SUMMARY OF THE INVENTION

The object of the invention is to provide a device such that the aboveproblems can be solved. This is achieved with the device in accordancewith the characterizing part of claim 1, which is characterized in thatdroplets atomized from at least one liquid starting material andcombustion gases and/or oxidizing gases forming a thermal reactor areintermixed prior to conducting the mixture into the thermal reactor.

The preferred embodiments of the invention are disclosed in thedependent claims.

The main object of the present invention is to provide a device by whichparticles having a size in the order of nanometers (1 to 1000 nm) can beproduced fast and economically. In particular, the object of theinvention is to provide a device by which it is possible to producemulticomponent nanoparticles.

In accordance with the invention the device uses liquid raw materialsthat are mostly solutions of metallic salts, the liquid is atomized intotiny droplets into a premixing chamber, mixed in the premixing chamberwith at least combustion gases, the mixture is conducted to a classifierthat separates the mixture for droplets having an aerodynamic diameterthat exceeds size d, the mixture containing droplets smaller than thediameter d are conducted to a burner, a flame is generated and in theflame the raw materials convert to nanoparticles whose composition maybe different from that of the raw materials.

In accordance with the invention, instead of the flame, it is alsopossible to use a thermal reactor other than the flame, such as plasma,hot-wall reactor, laser or the like, as the energy source required fornanoparticle formation, in which thermal reactor a premixed gas andliquid droplet mixture is conducted so as to form nanoparticles.

Further in accordance with the invention it is possible to feed into thepremixing chamber separately droplets of a variety of different rawmaterials and/or other nanoparticle raw materials in the form of a gasor vapour, whereby a raw material mixture of multicomponentnanoparticles will be obtained.

Further in accordance with the invention, the device comprises means foratomizing the liquid into droplets, means for conducting the dropletsinto a premixing chamber, means for conducting combustion and othergases into the premixing chamber, means for mixing the gases and theliquid droplets, means for removing large liquid drops from the mixture,means for conducting the mixture to at least one burner and means forgenerating a flame.

Further in accordance with the invention, surfaces of the device may beheated. In that case liquid from the droplets drifting onto the devicesurfaces evaporizes in the gas stream, but salts in the liquidcrystallize on the surfaces of the device and do not drift in the gasstream. In this manner it is possible to prevent the liquid deposited onthe surfaces from being detached in large drops in the gas stream.

The liquid droplets in the gas stream change size as a result ofcondensation and evaporation. When the diameter of a liquid dropletexceeds one micrometer, the liquid droplet always behaves approximatelyin the same manner, irrespective of whether salts are dissolved thereinor not (W. C. Hinds, Aerosol Technology, Properties, Behavior andMeasurement of Airborne Particles, 2nd Edition (1999), John Wiley &Sons, Inc. New York, in particular p. 278-303).

The change in droplet size for droplets of more than 1 micrometer may becalculated with formula

$\begin{matrix}{\frac{\mathbb{d}\left( d_{p} \right)}{\mathbb{d}t} = {\frac{4D_{v}M}{R\;\rho_{p}d_{p}}\left( {\frac{p_{\infty}}{T_{\infty}} - \frac{p_{d}}{T_{d}}} \right)}} & (1)\end{matrix}$where d_(p) is the diameter of a liquid droplet, t is time, D_(v) isdiffusion constant of vapour in the air, M is molecular mass of theliquid, R is gas constant, ρ_(p) is droplet density, ρ_(∞) is vapourpressure at a distance from the liquid droplet, T_(∞) is temperature ata distance from the liquid droplet, p_(d) is vapour pressure on thesurface of the droplet and T_(d) is temperature on the surface of thedroplet. If the right side of the equation has a negative value, thedroplet reduces. If the value is positive, the droplet becomes larger.

Droplet reduction rate is higher in small particles, i.e. as a result ofevaporation small particles disappear faster than greater ones anddroplet size distribution becomes wider. This will also result in saltcontained in small droplets drying in the channels, which may poseproblems for the process to work.

The liquid droplet size is also affected by their agglomeration, therate of which depends on the droplet density.

Gravitation has a substantial effect on the behaviour of droplets largerthan 50 micrometers in diameter. Typically, in the device of the presentinvention droplets larger than 50 micrometers are not produced into thepremixing chamber.

In one embodiment of the device of the invention aerosol flow from themixing chamber to the burner passes in channels, where evaporation doesnot occur from the liquid droplet surface, and consequently, dropletsize distribution of the droplets produced into the premixing chamberdoes not become wider as the droplets pass to the burner. Evaporation isprevented by controlling the gas temperature, the temperature of thedroplet-forming liquid, the surface temperature of the channels and thevapour pressure of the solvent (relative humidity when water is thesolvent).

In one embodiment of the device of the invention aerosol flow from themixing chamber to the burner passes in channels, where condensationtakes place on the surface of the liquid droplets, and consequently,droplet size distribution of the droplets produced in the premixingchamber becomes narrower as the droplets pass to the burner.Condensation is provided by controlling the gas temperature, thetemperature of the droplet-forming liquid, the surface temperature ofthe channels and the vapour pressure of the solvent.

The basic idea of the invention is to allow production ofmulticomponent, nanosized particles with the device of the invention inan industrial and scalable manner. Further, nanoparticles produced withthe device can be utilized in fabrication of a plurality of products,such as in coating or surface modification of glass or ceramic products,or in fabrication of fibre preforms to be produced in the manufacturingprocess of optical fibres.

In the following, the invention will be described in greater detail toillustrate by means of examples to a person skilled in the art somepreferred applications and advantages to be achieved by the inventionover the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described with reference to theattached drawings, wherein

FIG. 1 illustrates schematically the prior art,

FIG. 2 illustrates schematically a preferred embodiment of theinvention,

FIG. 3 illustrates an embodiment of the invention, in whichnanoparticles are produced by the device of the invention for dyeingflat glass,

FIG. 4 illustrates an embodiment of the invention, in whichnanoparticles are produced by the device of the invention for coating aceramic plate with a photocatalytic semi-conductor,

FIG. 5 illustrates an embodiment of the invention, in whichnanoparticles are produced by the device of the invention forfabrication of an optical fibre preform,

FIG. 6 illustrates an embodiment of the invention, in which the surfaceof the device is heated such that large drops are prevented from beingcarried to a downwardly directed burner, and

FIG. 7 illustrates the device of the invention, in which particularinterest has been paid to control the liquid droplet to be used ingenerating nanoparticles.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows prior art of producing nanosized particles, for instance,in the manner disclosed in Finnish patent FI98832. A liquid flame spray101 provides a flame 108 for spraying a substance to be sprayed. Thenecessary gases are supplied to the flame spray 101 through gas channels102, 103 and 104. Through the gas channels 102, 103 and 104 there aresupplied combustion gases generating the flame, a sprinkling gas of theliquid to be sprayed and optionally a gas provided for reaction control.The substance to be sprayed is introduced in liquid form into the flamespray 101 through a liquid channel 105. The liquid to be sprayed istransferred through the liquid channel 105 by pumping it with a spraypump 106. At an extremity of the flame spray there is a nozzle 107, inwhich the combustion gases are ignited to provide a flame and in whichthe liquid to be sprayed is formed into droplets by means of asprinkling gas. By means of a gas stream the liquid droplets areconducted to the flame 108, in which the liquid evaporates and metalcompounds in the substance to be sprayed form particles 109.

When nanosized particles are produced by the method of FIG. 1 a problemarises that large liquid drops that may be formed in the process do notevaporate completely, and there appears so-called residual particleshaving a size that exceeds the desired nanoparticle size. Anotherproblem with the method concerned is that it is difficult to produceparticles from liquid components that do not mix well together or thatreact with one another in an undesirable manner, forming a gel, forinstance.

FIG. 2 shows a preferred embodiment of the present invention. Liquiddroplets 202A are atomized into a premixing chamber 201 by using agas-dispersed atomizer 203, in which liquid feedstock 105A is atomizedby means of a combustion gas 104A into tiny droplets 202. Other liquiddroplets 202B are atomized into the same premixing chamber 201 by usinga pressure-dispersed atomizer 204, wherewith liquid feedstock 105B isatomized into tiny droplets 203. Further, third liquid droplets 205 areatomized into the same premixing chamber 201 by using an atomizer 206based on a vibrating plate, such as ultra-sound plate, wherewith theliquid source 207 is atomized into tiny droplets 205. Combustion gas isfed into the premixing chamber 201 from channel 104B. The combustion gasmay be hydrogen, methane, propane or butane or a combination thereof ora combination of these gases and some other gas. Likewise, anoxygen-carrying gas is fed into the premixing chamber 201 from thechannel 103. The oxygen-carrying gas may be air, oxygen or ozone. Aninert gas, such as nitrogen or carbon dioxide, is fed from the gaschannel 102 into the premixing chamber. A gas that contains at least oneraw material for nanoparticles to be produced is fed from the gaschannel 15 to the premixing chamber. Vapour that contains at least oneraw material for nanoparticles to be produced is fed from the channel208 into the premixing chamber. The vapour is introduced by feeding gasfrom the channel 209 through a bubbler bottle 211 containing liquid,whereby vaporized liquid (vapour) passes through the channel 208 intothe premixing chamber 201. Liquid droplets, gases and vapours mixefficiently in the premixing chamber 201 forming a homogeneous mixture212. The mixture is forwarded to a droplet separator 213 which separatesthe mixture for droplets 214 having a particle size exceeding size d.The liquid contained in the droplets is further conducted via acollector channel 215 into a collector container 216. The dropletseparator 213 may be based on, for instance, impaction, airclassification, electrical classification, cyclone-based classificationor the like. The mixture, from which large liquid drops are removed 217,is conducted to a burner 218. In the burner the mixture is ignited togenerate a flame 108. The flame is advantageously turbulent or otherwisesuch that the mixture will be efficiently mixed. In the flame 108 theliquid components evaporate and the raw materials react to formparticles 109.

FIG. 3 shows a preferred embodiment of the present invention whenapplied to dyeing of flat glass. At the same time, the figure and therelating description serve as an example of the use of the invention inone application. Liquid droplets 202A and 202B are atomized into thepremixing chamber 201 by using two pressure-dispersed atomizers 204A and204B, wherewith liquid feedstocks 105A and 105B are atomized into tinydroplets. The liquid 301A is sucked with a high-pressure pump 302A andfed further to the pressure-dispersed atomizer 204A. The liquid 301Aconsists of methanol and cobalt (II) nitrate Co(NO₃)₂6H₂O dissolvedtherein, the proportions being 100 ml methanol and 20 gcobalt(II)nitrate. The feed from the high-pressure pump 302A isrestricted with a throttle valve or the like 303A to be 50 ml/min. Theliquid 301B is sucked with the high-pressure pump 302B and fed furtherto the pressure-dispersed atomizer 204B. The liquid 301B consists ofmethanol and calcium nitrate Ca(NO₃)₂4H₂O dissolved therein, theproportions being 100 ml methanol and 18 g calcium nitrate. Feed fromthe high-pressure pump 302B is restricted with a throttle valve or thelike 3038 to be 50 ml/min. Hydrogen gas is fed into the premixingchamber 201 from the channel 104 at a volume flow rate of 500 l/min. Airis also fed into the premixing chamber 201 from the channel 103 at avolume flow rate of 1250 l/min. The liquid droplets and gases mixefficiently in the premixing chamber 201 and form a homogeneous mixture212. The mixture is conducted further to a droplet separator 213 thatseparates the mixture for droplets 214 having a diameter that exceedsthe aerodynamic diameter of 10 micrometers. The liquid contained in thedroplets is conducted further through a collector channel 215 to acollector container 216. The mixture, from which large liquid drops havebeen removed 217, is conducted to a burner 304. The burner 304 is a slitnozzle having a width of 1000 mm and a slit width of 20 mm. In theburner the mixture is ignited to generate a flame 108. In the flame 108the liquid components evaporate and the raw materials evaporate and/ornucleate and/or condense and/or react to form particles 109. Theseparticles are further directed to a surface of flat glass 305 having atemperature that exceeds 600° C. and to which the particles 306 adhere.The particles 306 diffuse and further dissolve in the glass 305 dyeingthe glass surface blue.

FIG. 4 shows a preferred embodiment of the present invention when it isused for producing a photocatalytic surface onto a ceramic plate. At thesame time the figure and the relating description serve an example ofthe use of the invention in one application. Liquid droplets 202A and202B are atomized into the premixing chamber 201 by using twogas-dispersed atomizers 203A and 203B wherewith liquid feedstocks 105Aand 105B are atomized into tiny droplets 202A and 202B. The liquid 301Ais fed with a hose pump 302A to the gas-dispersed atomizer 203A. Theliquid 301A consists of methanol and tetraethyl ortho titanateTi(OC₂H₅)₄ (TEOT) dissolved therein, the mixing proportions being 1:1.Volume flow rate of the liquid is regulated with the hose pump 302A tobe 10 ml/min. The liquid 301B is fed with the hose pump 302B to thegas-dispersed atomizer 105B. The liquid 301B consists of methanol andsilver nitrate AgNO₃ dissolved therein, the mixing proportions being 100ml methanol and 10 g silver nitrate. Volume flow rate of the liquid isregulated with the hose pump 302B to be 5 ml/min. Hydrogen gas is fedinto the premixing chamber 201 at the volume flow rate, regulated with aflow rate regulator 401, of 40 l/min from the channel 102 which furtherjoins with the gas channel of the gas-dispersed atomizer 203A prior tobeing conducted into the premixing chamber 201. In the same way, oxygenis fed into the premixing chamber 2011 at a volume flow rate, regulatedwith a flow rate regulator 402, of 20 l/min from the channel 103 whichfurther joins with the gas channel of the gas-dispersed atomizer 203Bprior to being conducted into the premixing chamber 201. The liquiddroplets 202A and 202B and the gases mix efficiently in the premixingchamber 201 and form a homogeneous mixture 212. The mixture is furtherconducted to a droplet separator 213 which separates the mixture fordroplets 214 having a diameter that exceeds the aerodynamic diameter of10 micrometers. Liquid contained in the separated droplets is furtherconducted through a collector channel 215 to a collector container 216.The mixture, from which large liquid drops have been removed 217, isconducted to a burner 304. The burner 304 is a slit nozzle having awidth of 300 mm and slit width of 5 mm. In the burner 304 the mixture isignited to generate a flame 108. In the flame 108 the liquid componentsevaporate and the raw materials react and form particles 109. Theseparticles are further directed onto the surface of a ceramic plate, onwhich they adhere forming a photocatalytic coating on the surface of theplate.

FIG. 5 shows a preferred embodiment of the present invention, when it isused for producing porous fibre preforms necessary for the manufactureof active optical fibres. At the same time the figure and the relatingdescription serve as an example of the use of the invention in oneapplication. Liquid droplets 202A and 202B are atomized into thepremixing chamber 201 by using two gas-dispersed atomizers 203A and 203Bwherewith liquid feedstocks 105A and 105B are atomized into tinydroplets. Compressed air is used as the atomizing gas 501A and 501B,which is conducted into gas channels 502A and 502B of the atomizingnozzle via flow rate regulators 401 and 402. The liquid 301A is fed witha hose pump 302A to the gas-dispersed atomizer 203A. The liquid 301Aconsists of methanol and aluminum nitrate Al(NO₃)₂9H₂O, the mixingproportions being 20 g aluminum nitrate to 100 ml methanol. Volume flowrate of the liquid is regulated with the hose pump 302A to be 12 ml/min.The liquid 301B is fed with the hose pump 302B to the gas-dispersedatomizer 203B. The liquid 301B consists of methanol and erbium nitrateEr(NO₃)₂5H₂O dissolved therein, the mixing proportions being 100 mlmethanol and 2 g erbium nitrate. Volume flow rate of the liquid isregulated with the hose pump 302B to be 12 ml/min. Oxygen 209 acting asa carrier gas is passed through the mass flow regulator 503 to a bubbler211. Silicon tetrachloride SiCl₄ 210 in the bubbler 211 vaporizes intothe carrier gas 209 and passes with the carrier gas 208 into thepremixing chamber 201. Flow rate of the carrier gas is 500 ml/min andthe temperature of the bubbler is 30° C. Hydrogen gas is fed into thepremixing chamber 201 from the channel 104 at a volume flow rate of 30l/min. In the same way, oxygen is fed into the premixing chamber 201 ata volume flow rate of 15 l/min from the channel 103. The droplets,vapour and gases mix efficiently in the premixing chamber 201 and form ahomogenous mixture 212. The mixture is further conducted to the dropletseparator 213 that separates the mixture for droplets 214 having adiameter that exceeds the aerodynamic diameter of 8 micrometers. Theliquid contained in the droplets is further conducted through thecollector channel 215 to the collector container 216. The mixture, fromwhich large liquid drops have been removed 217, is passed to a burner218. The burner 218 is a round nozzle having a diameter of 2 mm. In theburner 218 the mixture is ignited to generate a flame 108. In the flame108 the liquid components evaporate and the raw materials react and formparticles 109. The particle composition is SiO₂—Al₂O₃—Er₂O₃ theproportions being 100-10-1. These particles are further directed ontothe surface of a mandrel 504, on which the particles adhere and form aporous layer of glass. After build-up the mandrel 504 is removed, and aporous glass preform is obtained as a result.

FIG. 6 shows a preferred embodiment of the present invention when it isused in a downwardly directed burner for producing nanoparticles forcoating. At the same time the figure and the relating description serveas an example of the use of the invention in one application. Liquiddroplets 202A and 202B are atomized into the premixing chamber 201 byusing two gas-dispersed atomizers 203A and 203B wherewith liquidfeedstocks 105A and 105B are atomized into tiny droplets 202A and 202B.The liquid 301A is fed with the hose pump 302A to the gas-dispersedatomizer 203A. The liquid 301A consists of methanol and copper nitrateCu(NO₃)₂3H₂O dissolved therein, the mixing proportions being 30 g coppernitrate to 100 ml methanol. Volume flow rate of the liquid is regulatedwith the hose pump 302A to be 8 ml/min. The liquid is fed with the hosepump 302B to the gas-dispersed atomizer 203B. The liquid 301B istetraethyl ortho silicate TEOS. Volume flow rate of the liquid isregulated with the hose pump 302B to be 20 ml/min. Hydrogen gas is fedinto the premixing chamber 201 at a mass flow rate of 30 l/min from thechannels 103A and 103B which serve at the same time as disperse gaschannels of the gas-dispersed atomizers 105A and 105B. In the same way,air is fed into the premixing chamber 201 from the channel 501 at avolume flow rate of 75 l/min. The liquid droplets and gases mixefficiently in the premixing chamber 201 and form a homogeneous mixture212. The mixture is further conducted to a burner 304. The burner 304 isa slit nozzle having a width of 200 mm and a slit diameter of 1 mm. Thesurfaces 601A and 601B of the burner 304 and the premixing chamber 201are heated with electric heaters 602A and 602B to a temperature of 120°C. Thus methanol 603A and 603B evaporates from particles 605A and 605Bdrifted on the surfaces 601A and 601B, and salts 604A and 604B containedin the liquid adhere to the surfaces. Thus the liquid adhering to thesurface is prevented from flowing/running as large drops off the burner304. In the burner 304 the mixture is ignited to generate a flame 108.In the flame 108 the liquid components evaporate and the raw materialsreact and form particles 109. The particles are further directed to thesurface of flat glass 305, to which the particles 109 adhere. Theparticles diffuse, dissolve and mix in the surface of the flat glassdyeing it turquoise.

FIG. 7 shows schematically a device in accordance with the presentinvention, which device caters for several details affecting the controlof droplets to be produced. Liquid droplets 202 are atomized into apremixing chamber 201 with a pressure-dispersed atomizer 203. The liquid301, which contains metallic salts serving as raw materials fornanoparticles and a solvent thereof, is fed with a spray pump 106 into achannel 105. The liquid is atomized with the pressure-dispersed atomizer203 by means of hydrogen gas. Prior to introduction to a sprinkler 203the hydrogen gas is passed through a bubbler 717. The bubbler 717contains the same solvent 705 as the one in which the salts serving asnanoparticle raw materials have been dissolved. Solvent 705 vaporizes inthe hydrogen gas 103 passing through the bubbler 717. The amount ofvaporizing solvent depends on the volume flow of hydrogen and thetemperature of the bubbler 717. The temperature of the bubbler isadjusted by holding the bubbler 717 in a heat bath 704, the temperatureof which is controlled with a temperature controller 709. The hydrogengas vaporized by solvent is conducted to the atomizer 203 through thechannel 706. Oxidizing gas 104 is conducted to the premixing chamber 201through the channel 708. Upstream of the channel 708 the oxidizing gas104 is conducted through a bubbler 718. The bubbler 718 contains solvent707 that may the same as or different from the solvent 705. Solvent 707vaporizes in the oxidizing gas passing through the bubbler 718. Theamount of vaporizing solvent depends on the volume flow of the oxidizinggas and the temperature of the bubbler 718. Alternatively, hydrogen gasand oxidizing gas may be premixed with one another and the premixed gasmixture may be used for atomization of the liquid. This alternative isshown in FIG. 7B, in which the hydrogen gas passing in the channel 706Band the oxidizing gas passing in the channel 708B are combined in thechannel 710, before the gas mixture sprinkles the liquid passing in thechannel 105B. Liquid droplets 202 and gases 706 and 708 mix with oneanother in the premixing chamber 201. To enhance the mixing, thepremixing chamber 201 may comprise tumblers 715 which enhance the mixingof gas streams. A desired vapour pressure 716 of the solvent 705 and/or706 is generated in the premixing chamber 201. The premixing chamber 201may be heated or cooled with a heating/cooling jacket 711 that iscontrolled by a temperature controller 712. It is also possible tointroduce an extra gas stream from the channel 719 through a porous wall714 into the premixing chamber. With the gas stream conducted throughthe wall 714 it is possible to reduce accumulation of liquid particles202 on the walls of the chamber 201. An aerosol mixture exiting from thepremixing chamber is ignited to generate a flame 108. The dischargeopening 719 of the premixing chamber is arranged such that the flow rateof the gas discharged from the opening is higher than the flamepropagation rate in the discharging gas mixture, whereby the flame isprevented from burning in the premixing chamber 201. In the flame thedroplets and the metallic salts therein evaporate, react, nucleate,condense, agglomerate and/or sinter forming nanoparticles 109.

In accordance with the present invention it is further possible toprovide a method for producing nanoparticles. The method produces liquiddroplets containing nanoparticle starting materials, and the droplets(202A, 202B) are conducted to a thermal reactor, such as a flame, inwhich the nanoparticles (109) are generated from the droplets. In theinvention, liquid droplets atomized from at least one liquid startingmaterial and combustion and/or oxidizing gases constituting the thermalreactor are intermixed prior to conducting the mixture into the thermalreactor. The thermal reactor may be a flame generated by a combustiongas and an oxidizing gas or plasma provided by means of gas.

The median of the aerodynamic diameter of liquid droplets to beconducted into the flame is provided within the range of 0.1 to 50micrometers. In addition, liquid droplets having an aerodynamic diameterexceeding 5 to 50 micrometers are removed from the gas stream before theflame.

In the method, gas or vapour participating in at least one nanoparticlegeneration reaction is mixed into the aerosol particles and thecombustion and oxidizing gases providing the flame.

The liquid particles are atomized with a pressure-dispersed atomizer, agas-dispersed atomizer or a vibrating plate.

Naturally, it is obvious to a person skilled in the art that bycombining in a variety of ways the processes, methods and structuresdescribed above in connection with various applications of the inventionit is possible to provide different uses which fall within the spirit ofthe invention. Therefore the above examples shall not be understood torestrict the invention, but the embodiments of the invention may varyfreely within the scope of the inventive features disclosed in theaccompanying claims.

Naturally, it is also obvious to a person skilled in the art that theattached drawings are intended to illustrate the invention and thereforethe structures and components appearing therein are not drawn to correctmutual scale.

It is also obvious to a person skilled in the art that the presentedgeometries are only intended to illustrate the invention and thus, forinstance, the shape of the mixing chamber may be arbitrary and the shapeof the burner may be freely selected, provided that the geometriesemployed do not harmfully collect liquid particles, for instance. Aperforated burner is an example of a disadvantageous shape, in whichsurfaces between the perforations may act as impaction collectors ofparticles.

1. A device for producing nanoparticles, the device comprising means foratomizing liquid, wherein the means is configured to disperse liquidinto liquid droplets having the median of the aerodynamic diameterbetween 0.1-50 micrometers, the produced liquid droplets containingstarting materials for nanoparticles, said liquid droplets beingconducted to a thermal reactor, where the nanoparticles are generatedfrom the liquid droplets, wherein the liquid droplets and the combustionand/or oxidizing gases constituting the thermal reactor are intermixedprior to conducting the mixture to the thermal reactor, and the deviceis configured to conduct liquid droplets having the aerodynamic diameter5 micrometers or less to the thermal reactor.
 2. The device according toclaim 1, wherein the device comprises means for removing liquid dropletshaving an aerodynamic diameter that exceeds 5 to 50 micrometers from thegas stream upstream of the thermal reactor.
 3. The device according toclaim 2, wherein the device comprises an impactor, an electricclassifier, an air collector or the like for removing liquid dropletshaving an aerodynamic diameter that exceeds 5 to 50 micrometers.
 4. Thedevice according to claim 2, wherein the device comprises means forheating surfaces in contact with the mixture streams.
 5. The deviceaccording to claim 1, wherein the thermal reactor is a flame generatedby the combustion gas and the oxidizing gas.
 6. The device according toclaim 1, wherein the thermal reactor is plasma provided by means of gas.7. The device according to claim 1, wherein the device comprises meansfor heating walls of the device such that the liquid component ofdroplets drifting onto the wall will evaporate completely or partly andthe salt contained in the liquid adheres to the heated surface.
 8. Thedevice according to claim 1, wherein the device comprises means formixing at least one gas or vapour participating in a nanoparticlegeneration reaction with aerosol particles and the combustion andoxidizing gases generating the flame.
 9. The device according to claim1, wherein the device comprises means for vaporizing a solvent used inliquid droplets necessary for nanoparticle generation into at least onegas to be introduced into the premixing chamber of the device.
 10. Thedevice according to claim 1, wherein the device comprises apressure-dispersed atomizer, a gas-dispersed atomizer or a vibratingplate for atomizing liquid particles.
 11. The device according to claim1, wherein the device in provided such that the discharge rate of theaerosol exiting from the premixing chamber is higher than the flamefront propagation rate in said aerosol.
 12. A method for producingnanoparticles, wherein nanoparticles are produced with the device inaccordance with claim
 1. 13. An apparatus for dyeing a surface of flatglass, for manufacturing a photocatalytic surface, for manufacturing anoptical fiber perform, or for coating a ceramic, glass or metallicpiece, wherein the apparatus comprises the device according to claim 1.